1. Field of the Invention
The present invention relates to a vibration damping rubber composition, and more particularly, to a vibration damping rubber composition suitable for use in an engine mount and the like for supporting an engine of an automobile and the like, and also for restricting transmission of vibration of such an engine.
2. Description of the Art
Heretofore, a rubber composition, for example, obtained by adding a sulfur vulcanizing agent or a sulfur vulcanization accelerator to a rubber material such as natural rubber or synthetic rubber has been used as a vibration damping rubber composition. However, such a rubber composition tends to harden after long-term use (or after thermal aging) and becomes inferior in a vibration damping property.
It is thought that such thermal deterioration may be caused by the following reasons; (a) an increase of crosslinking density due to a change of polysulfide bonds to disulfide or monosulfide bonds; (b) oxidative aging of the rubber material; and (c) crosslinking due to a sulfur vulcanizing agent.
Countermeasures have been taken for the above-mentioned problems; by forming monosulfide-rich crosslinking for the above-mentioned problem (a); adding an antioxidant for the above-mentioned problem (b); and minimizing the blend amount of a sulfur vulcanizing agent for the above-mentioned problem (c). However, hardening of rubber cannot be totally restricted after long-term use (or after thermal aging), resulting in an inferior vibration damping property for the rubber.
Therefore, to solve these problems in terms of maintaining the vibration damping property, various rubber compositions using a bismaleimide compound as a vulcanizing agent have been proposed (see, for example, Japanese Unexamined Patent Publication Nos. 2-284935 and 4-136049, and Japanese Patent Publication No. 7-122006).
However, each of rubber compositions proposed by the above-mentioned publications uses a sulfur vulcanizing agent along with a bismaleimide compound. Therefore, the rubber composition tends to be disadvantageous in that a sufficient vibration damping property cannot be obtained because crosslinking proceeds by means of the sulfur vulcanizing agent so that the rubber becomes hard.
In view of the foregoing, it is an object of the present invention to provide a vibration damping rubber composition capable of exhibiting an excellent vibration damping property.